"Where is the jubilee taking place?" is another correct answer which Duo does not accept. In British English, "jubilee" is a common word and is used interchangeably with "anniversary".
It doesn't have to be a wedding anniversary (that would be specifically called bryllupsdag - "wedding day"). Might be on any occasion you can have an anniversary for. 20 years of employment at the same workplace, for instance.
"Where does the anniversary happen?" is not accepted.
Are "happen" and "take place" so different?
Well, what is interesting in translating "at finder sted" by "to take place", is that it keeps the same half of the expression: "sted" and "place". I realize this now.